Chapter 6: The Muscular System Skeletal Muscle involuntary. Important key words for this muscle type are cardiac, striated, and involuntary. Endomysium attached to the fibrous skeleton of the heart. Cover our bone and cartilage framework, they help form the smooth contours of the body. Single, very long, cylindrical, multinucleate cells with very obvious striations. Skeletal muscle is also known as striated muscle (because its fibers have obvious stripes) and as voluntary muscle (because it is the only muscle type subject to conscious control). Epimysium, perimysium, and endomysium. When you think of skeletal muscle tissue, the key words to remember are skeletal, striated, and voluntary. Smooth Muscle a. Produce Movement - Skeletal muscles are responsible for our body’s mobility, including all locomotion and manipulating things with your agile upper limbs. - They enable us to respond quickly to changes in the external environment. b. Maintain Posture and Body Position - has no striations and is involuntary, which means that we cannot consciously control it. Found mainly in the walls of hollow (tubelike) visceral organs such as the stomach, urinary bladder, and respiratory passages, smooth muscle propels substances along a pathway. Smooth muscle fibers are spindle-shaped, uninucleate, and surrounded by scant endomysium. Cardiac Muscle found in only one place in the body—the heart, where it forms the bulk of the heart walls. cardiac muscle is striated, and like smooth muscle, it is uninucleate and its control is We are rarely aware of the workings of the skeletal muscles that maintain body posture. Yet they function almost continuously, making one tiny adjustment after another so that we maintain an erect or seated posture. c. Stabilize Joints - Muscles and tendons are extremely important in reinforcing and stabilizing joints that have poorly articulating surfaces, such as the shoulder and knee joints. d. Generate Heat - Muscle activity generates body heat as a byproduct. As ATP is used to power muscle contraction, nearly three-quarters of its energy escapes as heat. characteristics include mal functioning mitochondria, inflammation, and the generation of free radicals that damage DNA and tissue much like intense UV light. Microscopic Anatomy of Skeletal Muscle • • • • • Sarcolemma- oval nuclei can be seen just beneath the plasma membrane. Sarcomeres- myofibrils are chains of tiny contractile units. The sarcomeres are aligned end to end like boxcars in a train along the length of the myofibrils • • • • • • • • • • Thick filaments- mostly of bundled molecules of the protein myosin, but they also contain ATPase enzymes, which split ATP to release the energy used for muscle contraction. Thin filaments- composed of the con tractile protein called actin, plus some regulatory proteins that play a role in allowing (or preventing) binding of myosin heads to actin. Sarcoplasmic reticulum (SR)- —is a specialized smooth endoplasmic reticulum and store calcium and to release it on demand when the muscle fiber is stimulated to contract. Skeletal Muscle Activity • Contraction of a Skeletal Muscle as a Whole • Three pathways for ATP regeneration during muscle activity. 1. Direct phosphorylation of ADP by creatine phosphate- The unique high-energy molecule creatine phosphate (CP) is found in muscle fibers but not other cell types. As ATP is depleted, interactions between CP and ADP result in transfers of a high-energy phosphate group from CP to ADP, thus regenerating more ATP in a fraction of a second. 2. Aerobic respiration- occurs in the mitochondria and involves a series of metabolic pathways that use oxygen. During aerobic respiration, glucose is broken down completely to carbon dioxide and water, and some of the energy released as the bonds are broken is captured in the bonds of ATP molecules. 3. Anaerobic glycolysis and lactic acid formationmetabolic pathway that breaks down glucose into pyruvic acid without using O2 to generate ATP. Irritability- also termed responsiveness, which is the ability to receive and respond to a stimulus. Contractility- is the ability to forcibly shorten when adequately stimulated. Extensibility- is the ability of muscle fibers to stretch. Elasticity- is their ability to recoil and resume their resting length after being stretched. Motor unit- consists of one neuron and all the skeletal muscle fibers it stimulates. Axon- long, threadlike extension of the neuron. Neuromuscular junctions- the junction of a motor neuron’s axon terminals and the sarcolemma of a muscle cell. Neurotransmitter- a chemical substance released by a neuron when the nerve impulse reaches its axon terminals. Acetylcholine- specific neurotransmitter that stimulates skeletal muscle fibers. ALS, Or Amyotrophic Lateral Sclerosis (Lou Gehrig’s Disease)- motor neurons degenerate over time, resulting in paralysis that gradually worsens. The cause of ALS is unknown, but common Graded Responses- Different degrees of contraction in response to different levels of stimulation (changes in both the stimuli frequency and number of muscle cells stimulated). Unfused tetanus/ Incomplete tetanus - When the muscle is stimulated so rapidly that no evidence of relaxation is seen and the contractions are completely smooth and sustained. Muscle Fatigue and Oxygen Deficit • Muscle fatigue- when a muscle is unable to contract even though it is still being stimulated. Suspected causes are imbalances in ions (Ca2+, K+) and problems at the neuromuscular junction. • Oxygen deficit- is not a total lack of oxygen; rather, it happens when a person is not able to take in oxygen fast enough to keep the muscles supplied with all the oxygen they need when they are working vigorously. a. Flexion- is a movement, generally in the sagittal plane, that decreases the angle of the joint and brings two bones closer together. b. Extension- it is a movement that increases the angle, or distance, between two bones or parts of the body (straightening the knee or elbow). c. Rotation- is movement of a bone around its longitudinal axis. d. Abduction- is moving a limb away (generally on the frontal plane) from the midline, or median plane, of the body. e. Adduction- is the opposite of abduction, so it is the movement of a limb toward the body midline. f. Muscle Tone- state of continuous partial contractions of muscles. Circumduction- is a combination of flexion, extension, abduction, and adduction commonly seen in ball-and-socket joints, such as the shoulder. g. If the muscles loses tone, it becomes flaccid, or soft and flabby, and begins to atrophy (waste away). This is called flaccid paralysis. Dorsiflexion- Lifting the foot so that its superior surface approaches the shin (pointing your toe toward your head). h. Plantar flexion- pointing the toes away from your head. i. j. k. Inversion- turning the sole medially. l. Pronation- occurs when the forearm rotates medially so that the palm faces posteriorly (or down). Types of Muscle Contractions • • • • Isotonic contractions- literally, “same tone” or tension. In isotonic contractions, the myofilaments are successful in their sliding movements, the muscle shortens, and movement occurs. Isometric contractions- literally, “same measurement” or length. In isometric contractions, the myosin filaments are “spinning their wheels,” and the tension in the muscle keeps increasing. They are trying to slide, but the muscle is pitted against some more or less immovable object. Effect of Exercise on Muscles • • • • Types of Body Movements Muscle inactivity (due to a loss of nerve supply, immobilization, or whatever the cause) always leads to muscle weakness and wasting. Regular exercise increases muscle size, strength, and endurance. Aerobic exercise or Endurance exercise- results in stronger, more flexible muscles with greater resistance to fatigue. Aerobic exercise helps us reach a steady rate of ATP production and improves the efficiency of aerobic respiration. Resistance exercise or Isometric exercise- pit the muscles against an immovable (or difficult to move) object. The increased muscle size and strength that result are due mainly to enlargement of individual muscle fibers (they make more contractile myofilaments) rather than to an increase in their number. Eversion- turning the sole laterally. Supination- occurs when the forearm rotates laterally so that the palm faces anteriorly (or up) and the radius and ulna are parallel, as in anatomical position. Facial Muscles • • Frontalis- covers the frontal bone, runs from the cranial aponeurosis to the skin of the eyebrows, where it inserts. This muscle allows you to raise your eyebrows, as in surprise, and to wrinkle your forehead. Orbicularis Oculi- run in circles around the eyes. It allows you to close your eyes, squint, blink, and wink. • • • • • Orbicularis Oris- is the circular muscle of the lips. It closes the mouth and protrudes the lips. Buccinator- runs horizontally across the cheek and inserts into the orbicularis oris. It flattens the cheek (as in whistling or blowing a trumpet). It is also listed as a chewing muscle. Zygomaticus- extends from the corner of the mouth to the cheekbone. It is often referred to as the “smiling” muscle because it raises the corners of the mouth. because it acts as a synergist to help tailors sit with both legs crossed in front of them. • Quadriceps Group- consists of four muscles— the rectus femoris and three vastus muscles. The group as a whole acts to extend the knee powerfully, as when kicking a soccer ball. Because the rectus femoris crosses two joints, the hip and knee, it can also help to flex the hip. Muscles Causing Movement at the Ankle and Foot • Masseter- covers the angle of the lower jaw. This muscle closes the jaw by elevating the mandible. Temporalis- is a fan-shaped muscle overlying the temporal bone. It inserts into the mandible and acts as a synergist of the masseter in closing the jaw. • Neck Muscles • • Platysma- is a single sheetlike muscle that covers the anterolateral neck. Its action is to pull the corners of the mouth inferiorly, producing a downward sag of the mouth (the “sad clown” face). Sternocleidomastoid- two headed muscles, one found on each side of the neck. When both sternocleidomastoid muscles contract together, they flex your neck. It is this action of bowing the head that has led some people to call these muscles the “prayer”. Muscles Causing Movement at the Knee Joint • • Hamstring Group- muscles forming the muscle mass of the posterior thigh are the hamstrings. The group consists of three muscles—the biceps femoris, semimembranosus, and semitendinosus. They are prime movers of thigh extension and knee flexion. Sartorius- It runs obliquely across the thigh from the anterior iliac crest to the medial side of the tibia. It is a weak thigh flexor. The sartorius is commonly referred to as the “tailor’s” muscle • • • Tibialis Anterior- It arises from the upper tibia and then parallels the anterior crest as it runs to the tarsal bones, where it inserts by a long tendon. It acts to dorsiflex and invert the foot. Extensor Digitorum- the extensor digitorum longus muscle arises from the lateral tibial condyle and proximal three-quarters of the fibula and inserts into the phalanges of toes 2 to 5. It is a prime mover of toe extension. Fibularis Muscles- They arise from the fibula and insert into the metatarsal bones of the foot. The group as a whole plantar flexes and everts the foot, which is antagonistic to the tibialis anterior. Gastrocnemius- is a two-bellied muscle that forms the curved calf of the posterior leg. It is a prime mover for plantar flexion of the foot; for this reason it is often called the “toe dancer’s” muscle. Soleus- it arises on the tibia and fibula (rather than the femur), it does not affect knee movement, but like the gastrocnemius, it inserts into the calcaneal tendon and is a strong plantar flexor of the foot.
